This invention relates to ion thrusters and, more particularly to the structure and material of construction of the grids used in the ion-optics system of the ion thruster.
Ion thrusters are used in spacecraft such as communications satellites for stationkeeping and other functions. An important advantage of the ion thruster over an engine using chemical propellants is that it utilizes the electrical power generated by the solar cells of the satellite to achieve the propulsion. The ion thruster requires relatively small amounts of a consumable propellant that is ionized, and it is not necessary to lift large masses of chemical fuel to orbit. The ion thruster also has a high specific impulse, making it an efficient engine which requires very little propellant.
In an ion thruster, a plasma of ions and electrons is created and confined within the body of the thruster. Ions from the plasma are electrostatically accelerated rearwardly by an ion-optics system. The reaction with the spacecraft drives it forwardly, in the opposite direction. An ion thruster is often built to be small in size, so that the force produced by the ion thruster is small. The ion thruster is therefore operated for a relatively long time. Consequently, for some missions, the ion thruster must be operable and reliable for thousands of hours of operation.
The ion-optics system includes grids to which appropriate voltages are applied in order to accelerate the ions rearwardly. The grids include aligned apertures therethrough. Some of the ions accelerated by the applied voltages pass through the apertures, providing the propulsion. Others of the ions impact the grids, heating them and etching away material from the grids. The heating and electrostatic forces on the grids combine to cause substantial mechanical forces at elevated temperature on the grids, which distort the grids and make them susceptible to fatigue and mechanical failure. These effects are taken into account in the design of the grids, so that they remain functional for the required extended lifetimes.
At the present time, the grids are typically made of molybdenum formed into a domed shape. It has also been proposed to make flat grids of carbon-carbon composite material. Both of these materials have limitations in respect to the lifetime of the grid and its mechanical strength. Accordingly, there is a need for a better material of construction and design for the grids of the ion-optics systems of ion thrusters. The present invention fulfills this need, and further provides related advantages.